Dissociation of peripheral protein‐membrane complexes by high pressure

The ability of high pressure to dissociate several peripheral protein‐membrane complexes was investigated. Three vitamin K‐dependent proteins (factor X, protein Z, and prothrombin) dissociated from small unilamellar vesicles (SUVs, 30 nm diameter) composed of 25% phosphatidylserine (PS) and 75% phosphatidylcholine (PC) at comparable pressures (midpoints of 0.3–0.6 kbar). The pressure‐induced dissociation curves for the factor X‐SUV interaction followed the expected behavior for an interaction with an apparent dissociation equilibrium constant at atmospheric pressure, K_D(atm), of 9 × 10⁻⁷ M and a change in volume of association, ΔV_a, of 88 mL/mol. Factor X also dissociated from large unilamellar vesicles (LUVs, 100 nm diameter, 25% PS:75% PC) with a midpoint of 0.5 kbar. A second group of calcium‐dependent membrane‐binding proteins included protein kinase C (PKC), a 64‐kDa protein, and a 32‐kDa protein. The 32‐kDa protein dissociated from SUVs (midpoint of 0.8 kbar), whereas PKC and the 64‐kDa protein did not dissociate to a significant degree. The differences in dissociability of these proteins appeared to be a result of the differences in their K_D(atm)'s (decreased dissociability with decreased K_D(atm)). This pattern was further demonstrated by the relatively high midpoint of dissociation (1.1–1.4 kbar) of serum amyloid P component (SAP; K_D(atm) ca. 10⁻¹¹) and the limited dissociation of factor V_a light chain (K_D(atm) ca. 10⁻¹¹). Changing the vesicle composition to phosphatidylethanolamine in place of PC gave higher affinity and decreased dissociation of the 32‐kDa protein and SAP. Thus, the susceptibility of various peripheral membrane‐binding proteins to high pressure dissociation was governed by both the K_D(atm) and the ΔV_a of the particular protein‐membrane interaction, with the ΔV_a's of the dissociable proteins being relatively small (< 150 mL/mol).